The authors had prev;ously shown (LSD I0 and LSD 15) that LSD caused an increase in the concentration of hexosemonophosphate (HMP) in the blood of normal subjects. In 13 schizophrenics LSD (80 mcg. orally) produced minimal psychological effects and an increase in blood HMP. In control studies, water caused a decrease in blood HMP in 9 schizophrenics. Mescaline given to 9 normal subjects produced psychological effects similar to those of LSD but caused no changes in blood chemistry. Studies on guinea pig tissue brei in the Warburg apparatus showed that LSD stimulated the respiration of brain tissue and decreased the breakdown of HMP in brain and liver tissue.